Liquids have been stored for many years in metal tanks and vessels at ambient temperature. In recent years there has been substantial interest in storing thermal energy in the form of a hot liquid, such as hot water, oil or a molten salt, for subsequent use as needed or appropriate. The energy so stored can be obtained from a power generating plant for subsequent electric generation or it can be obtained from a solar energy collector and used later for heating purposes or electric power generation.
A thermal energy storage tank will generally always be full of liquid. The liquid, however, will be removed hot from the tank when energy is needed and after the heat is removed the cold liquid will be returned to the tank for storage. The liquid in the tank will accordingly be at two different temperatures a substantial amount of the time with a cold layer stratified beneath an upper hot layer. When thermal energy is available to heat the liquid, the cold liquid is removed from the lower part of the stored volume, heated and then returned to the upper part of the tank. In this way the entire stored volume can be heated. Thus, the volume ratios of the cold liquid layer to the total volume can vary between 0 and 1.
Although spherical tanks can be used they are costly. A clear cost advantage is inherent in a flat bottom cylindrical tank, principally because the load can be transferred directly to the supporting earth.
Unless the storage tank is suitably insulated, substantial heat would be lost by convection, conduction and radiation from the hot liquid to the surrounding air. The insulation is desirably placed on the outside of the tank because functionally suitable insulation for use on the inside tank wall may not be available or, if available, the cost would be too great. The result of external insulation is that the hot liquid directly contacts the tank side wall. While such direct liquid contact is acceptable with moderately heated liquids, it is undesirable to have high temperature liquids, such as molten salts above 400.degree. C., in contact with the tank metal shell. At a moderately high temperature, the weakening effect on the wall strength is countered by increasing the design thickness of the shell but for higher temperatures (&gt;700.degree. C.) the weakening effect can create a hazardous condition. Also, severe temperature stresses are produced in the metal side wall or shell as a result of the thermocline where the hot and cold layers merge, and from the temperature difference between the cylindrical shell and bottom.
To minimize capital costs, it is desirable to fabricate a storage tank from the least expensive metal. However, molten salts at high temperatures, i.e. above 550.degree. C., are very corrosive to carbon steel and even stainless steels. While a layer of expensive refractory insulation on the inside of the tank might be considered a way to keep the metal shell at a lower temperature and out of contact with it, such an arrangement places the tank in jeopardy if a hole or fracture in the insulation permits the hot liquid to contact and heat the metal tank shell. Furthermore, many insulation materials are readily corroded, and thus are not useable, when placed in direct contact with a molten salt.
From the above discussion it is clear that a need exists for an improved storage tank for storing thermal energy in the form of a high temperature liquid, particularly a molten salt.